1. Field of the Invention
The present invention pertains to a vapor-compression refrigeration system particularly adapted for multi-story buildings wherein the compressor lubricating and/or cooling oil separated from the refrigerant fluid downstream of the compressor is collected at the base of a refrigerant vapor return riser and recirculated to the compressor by an automatically controlled oil pumping system.
2. Background Art
In the art of refrigeration and air conditioning systems for single and multi-story buildings and other structures, the use of vapor-compression systems with direct expansion of the refrigerant to evaporator or cooling units at various levels within the building has become particularly attractive due to overall economy of installation and superior performance. In the installation of vapor-compression refrigeration systems it has substantially always been a problem to deal with quantities of compressor lubricating and/or cooling oil which inevitably become entrained with the refrigerant fluid being discharged from the compressor. Even with oil separation devices interposed in the compressor discharge line at least minute quantities of oil remain entrained with the refrigerant fluid and, over long periods of time, collect in the system piping throughout the refrigerant circuit--including in particular, the refrigerant vapor return line from the evaporator to the compressor inlet. This oil must be recirculated back to the compressor in order to prevent unwanted accumulation in the refrigerant lines, and in installations involving substantial line length, it is important to be able to continuously recirculate the lubricating oil back to the compressor in order to minimize the total quantity of oil needed for compressor lubrication and/or cooling.
Providing for the recirculation of oil entrained in the refrigerant flow circuit of vapor-compression air conditioning systems in multistory buildings has been a particularly vexatious problem and has been dealt with to some extent in prior art installations by providing complex refrigerant vapor return riser systems leading from the evaporator units to the roof mounted compressor and condensing unit and which have oil traps disposed at various elevations throughout the length of the return risers. By providing oil traps in the form of reverse bends in the return risers, the oil is allowed to collect in the traps until the velocity of the refrigerant vapor flowing through the risers is sufficient to entrain the oil, carry it upwardly to the next trap, and so on until the oil is returned to the compressor unit. The complex and expensive plumbing arrangement of prior art refrigerant return risers not only makes this method of oil return expensive, but as a practical matter, the overall vertical elevation for which vapor-compression refrigeration systems may be utilized with direct expansion to the evaporator or cooling coil units at various levels has been limited to approximately four stories or 45-50 feet in overall elevation.
Moreover, the sizing of the refrigerant vapor return riser which is required to maintain sufficient gas velocity to entrain the liquid oil has been such that pressure losses in the system require greater compressor power input for a given performance level of the refrigeration system.
A still further problem with prior art refrigeration or air conditioning systems for multi-story buildings wherein the aforementioned type of refrigerant return riser is utilized pertains to maintaining sufficient velocity of the refrigerant vapor at reduced load conditions. In this regard, it has been necessary in many installations to provide secondary refrigerant vapor return lines which bypass the main riser oil traps so that return flow of refrigerant fluid is possible at reduced operating load and refrigerant fluid flow conditions. Secondary return risers have compounded the problems associated with the cost and complexity of vapor-compression refrigeration systems installed in multi-story as well as single story buildings and other installations. However, in accordance with the present invention, the complex and expensive piping systems of prior art direct expansion vapor-compression refrigeration or air conditioning systems have been eliminated with an improved arrangement of the refrigerant vapor return riser system and a unique method and apparatus for returning oil which separates from the refrigerant as it flows through the circuit from the evaporator back to the compressor.